The present application relates generally to compressors, including those used in heating, ventilation, air conditioning and refrigeration (“HVAC&R”) applications. More particularly, the present application relates to a pressure equalization system for starting a compressor, such as a scroll, rotary, or reciprocating compressor, while maintaining the condenser at a high pressure.
A standard HVAC&R system includes a fluid, an evaporator, a compressor, a condenser, and an expansion valve. In a typical refrigeration cycle, the refrigerant fluid begins in a liquid state under low pressure. The evaporator evaporates the low pressure liquid as the liquid absorbs heat from the evaporator, which raises the ambient temperature of the liquid and causes the liquid to undergo a phase change to a low pressure gas. The compressor draws the gas in and compresses it, producing a high pressure gas. The compressor then passes the high pressure gas to the condenser. The condenser condenses the high pressure gas to release heat to the condenser and the gas undergoes a phase change to a high pressure liquid. The cycle is completed when the expansion valve expands the high pressure liquid, resulting in a low pressure liquid. By means of example only, the refrigerant fluid used in the system might be ammonia, ethyl chloride, CFCs, HFCs, Freon®, or other known refrigerants.
Typically, upon start up of a compressor, the pressure at both the suction port and the discharge port of the compressor is low. In operation, the compressor works the fluid to achieve a high pressure at the discharge port. However, when the compressor is no longer operating, the fluid on the high pressure side of the compressor (toward the condenser) flows back toward the low pressure side of the compressor (toward the evaporator) until a state of equilibrium between the formerly high and formerly low pressure sides is achieved. Thus, the pressure tends to equalize between the low pressure side and the high pressure side when the compressor stops operating. Such a system is inefficient because the refrigeration cycle requires energy at start up to create a high pressure in the condenser, which is needed to condense the fluid.
Another problem, specific to HVAC&R systems, is that it is difficult to efficiently achieve the high pressure start up, i.e., a start up where the pressures have not equalized, necessitated by seasonal energy efficiency requirements (SEER), a system used to rate HVAC&R systems. Start up components, such as a start capacitor and a start relay, are commonly used to overcome the differential pressure when the compressor needs to start with the unbalanced pressure in the system, i.e., the high pressure side of the system has a high pressure and the low pressure side of the system has a low pressure. These components achieve a high pressure differential start when the system is activated. These components are rather expensive, however, and they produce high voltages and currents in the compressor motor upon start up.
Therefore what is needed is a system and method for equalizing the pressure in the compressor in order to start the compressor while maintaining a high pressure in the condenser and the high pressure portion of the system.